Several types of spectroscopic detectors are known, such as the Fourier transform type.
A two-dimensional spectrometer in integrated optics is known from patent application FR 2,889,587. In this application, the two interference beams are two counter-propagating waves introduced in a waveguide. Sampling tools in the form of for example nanostuds positioned along the waveguide allow for a spatial sampling of the evanescent waves delivered from this interference field. The detector is mono-dimensional and enables the capturing of the interference lines of the interferogram along their entire width, which makes the control of the detector's resolution possible.
The disadvantage of the described solution in this application lies in the limitation of the spectral band of the detector's analysis. This limitation is due to the distance between the nanostuds which cannot go lower than a certain value. In fact, below this value, problems of light scattering prevent getting satisfactory measures by spectrometer. Thus, the described system does not enable piecing together of the whole spectrum, by means of inverse Fourier transform, as the interferogram is sub-sampled, by loss of high frequency components.
Besides, a complementary solution to the one described above consists in using an optical element, such as a projection lens, in order to increase the interferogram size—and consequently of the interference lines—at the detector. For this, the interferogram is projected on a screen by conjugation with the lens. This way it is possible to adjust the resolution via the lens expansion, to sub-sample the interferogram. Consequently, the limitation of the spectral band by the effects of light scattering is reduced.
However, this solution requires placing a lens between the detector and the refracting surface and to move the detector away significantly. As a result, such a spectrometer is extremely voluminous.
Finally, compact spectroscopic detectors are known from the US patent application 2002/0075483. In this application, standing waves are sampled by an ultra-thin detector composed of a vibrating membrane. A mirror placed downstream from the detector reflects the light from a light beam having crossed through the detector so that it superimposes with the one from the same light beam and reaching this detector. The interferogram is then located in the central plane of the membrane. The latter vibrates so as to scan at least one part of the interferogram, which enables to increase the spectral resolution.
A disadvantage of this solution lies however in the limitation of the amplitude of the membrane vibration. This limitation is due to the necessary curvature of the membrane beyond a certain vibration amplitude, thus distorting the spectral measure significantly. Yet, since it is the vibration amplitude that determines the resolution, it appears that such a spectrometer is limited in spectral resolution.
Still other solutions exist to produce a compact spectroscopic detector, particularly Bragg's array dispersion detectors or stationary and dynamic Michelson's interferometers. However, these detectors do not obtain a high resolution without being necessarily limited in terms of analysis spectral band or being voluminous.
Thus all the solutions of the state of the art require a necessary compromise between, on the one hand, the spectral resolution and, on the other hand, the analysis spectral band and the consistency.